As a technique for producing propylene oxide by carrying out an epoxidation reaction of propylene with hydrogen peroxide in the presence of a crystalline titanosilicate catalyst, a method of using a TS-1 catalyst has been known and it has been well known that a methanol solvent is preferred in that case (for example, Journal of Catalysis 129, 159, (1991)). In order to obtain propylene oxide from the reaction mixture, however, propylene oxide must be separated by distillation from water and a methanol solvent, and additionally in order to recover the methanol solvent, water and methanol must be separated by distillation, which had a problem in that a huge amount of energy was required. Further, methanol and water were known to readily react with propylene oxide to produce methoxypropanol and propylene glycol, and hence there was a concern about a loss of propylene oxide. There was disclosed a process, therefore, (for example, in WO99/14208) in which the desired epoxy compound is separated from water solution by liquid-liquid extraction from a reaction mixture obtained by using a TS-1 catalyst and a methanol solvent with the use of an organic solvent capable of being separated from water solution. Since, however, another extraction solvent, which was different from the reaction solvent, was used in this process, there was a problem in that complicated steps such as distillation separation of two organic solvents and purification of each of the solvents by distillation were required. When the methanol solvent was not used, complicated steps were unnecessary but high activity was not attained.
Also, there were known processes using a Ti-MWW catalyst and an organic solvent such as methanol, acetone, acetonitrile or the like that can be completely mixed with water, and it was known that the acetonitrile solvent was preferred in that case and high activity was obtained as compared with the case of using a TS-1 catalyst and a methanol solvent (for example, 2001-fiscal year Next-generation Chemical Process Technical Development/Nonhalogen Chemical Process Technical Development Progress Report, 168-210, (2002)). However, there was a problem in that produced water and the solvent must be separated, and industrial distillation separation was huge energy consuming.